The present invention relates to a composite material having utility in electrical and electronic applications and to a process of forming said composite material.
Electronic packaging demands heat dissipating materials that can provide both good thermal conductivity and low thermal expansion. A low thermal expansion, matching that of semi-conductor and ceramic elements, is needed to avoid harmful stresses during thermal cycling of the electronic package.
Heat dissipating elements are frequently fabricated from strip material because of the required geometry of the element. A flat wafer, shallow tub, or lead frame geometry are common examples. It is important in many of these applications that the through-thickness thermal conductivity be maximized, while the in-plane thermal conductivity is of less significance.
Through-thickness conductivity is a function of the morphology of the low thermal conductivity phase of the composite. In particular, the aspect ratio of the low conductivity phase determines the thermal path length through the high conductivity matrix component from one side of the strip material to the other. As used herein, the term "aspect ratio" means the ratio of the mean in-plane dimension to the through-thickness dimension. It is thus desirable for the low conductivity phase material to have a low aspect ratio.
The processing of the composite strip is significant from the standpoint of maintaining a low conductivity phase with a low aspect ratio in the composite material. Most strip materials are rolled to a desired final thickness. In the case of materials having a deformable low conductivity phase material, rolling will increase the aspect ratio of this phase material. It is therefore desirable that the composite strip material be fabricated with the least amount of rolling.
Roll compaction processes are quite attractive in that they allow near net shape strip fabrication. The initial green strip thickness may be selected so that processing to full density also produces the required finished thickness, preferably without any additional rolling. The roll compaction process is particularly advantageous in forming composite strip where through-thickness thermal conductivity is to be optimized by providing a microstructure in which adjacent low thermal conductivity particles are not in contact.
Another problem which must be dealt with during processing is the closeness of the low conductivity phase particles. Close particles lead to high thermal resistance regions and may result in poor mechanical bonding to the matrix which has a deleterious effect on thermal expansion. To avoid these problems, individual low conductivity particles have been coated with the matrix material prior to the blending portion of the roll compaction process.
U.S. Pat. No. 4,894,293 to Breit et al. illustrates a process for forming a composite material in which the low expansion phase material is coated with the matrix material. In this process, discrete particles of a ferrous alloy are coated with a copper material. Coating of the ferrous alloy particles is effected by electroplating, electroless copper plating or cutting short lengths of a copper-clad fine wire of the ferrous alloy material. The coated particles are then pressed together and heated to diffusion bond the copper coatings. In this process, the heating step must be strictly regulated with respect to temperature and duration so that the copper coatings are diffusion bonded to each other while diffusion of the ferrous components into the copper material is avoided.
The prior art methods for forming a coating on the low thermal conductivity materials suffer from disadvantages such as: (1) difficulty in obtaining a uniform coating; (2) entrapment of impurities; (3) formation of particle agglomerates; and (4) disposal of waste chemicals.
Accordingly, it is an object of the present invention to provide an improved composite material for use in electrical and electronic applications.
It is a further object of the present invention to provide a composite material as above having improved thermal conductivity properties.
It is yet a further object of the present invention to provide a novel process for forming the composite material of the present invention.
These and other objects and advantages will be described in the following description.